There is a significant need in the art for improved secondary (rechargeable) batteries having high energy density, decreased cost, improved safety, reduced thermal management and improved stability of component supply. Batteries having such improved characteristics will be useful in a variety of applications and are of particular interest for electric utility grid storage.
Electric utilities are seeking more cost effective and efficient strategies to manage energy delivery to the grid. Peak demand is frequently met by the use of relatively expensive gas turbines, which at times of low demand remain idle. Ideally, base load electrical energy production could be operated at optimum peak efficiency, with demand variations being either absorbed or delivered using some form of energy storage. Pumped hydro (PH) technology, where water is reversibly pumped from a lower to higher elevation, has been employed for such energy storage, where round-trip efficiency is typically 68%. However, acceptable sites for implementation of PH energy storage, based upon location and environmental concerns, are now very limited. An alternatively is the use of large storage batteries, where round-trip energy conversion efficiencies can exceed that of PH, and wherein siting is not limited by geography. The market for storage batteries for this application is expected to grow, provided that battery costs are reduced and performance is increased. Major issues that are currently limiting implementation of advanced battery systems for grid storage include: overall cost for materials and associated hardware, long-term availability of materials, safety, achieving long cycle life and 5) thermal management during operation. The present invention provides an improved alkali metal/oxygen battery to meet these requirements. The batteries of the invention incorporate no toxic materials, and are generally safer than comparable battery systems (e.g., sodium-sulfur systems).
Improved secondary batteries will also provide particular benefit for applications to electric vehicles and their use will translate into greater range for such vehicles.
Various secondary metal air batteries have been reported. Cells of these batteries contain an air cathode (positive electrode) and a metal-based anode (negative electrode) separated by a liquid or solid electrolyte.
WO 2012/061817 relates to a secondary alkali-metal-air battery having a porous air cathode made of porous carbon with an electrocatalyst. The battery has an anode comprising alkali metal and a lithium ion-conductive membrane (e.g., a glass ceramic). The electrolyte is described as a fluid which is circulated between the anode and cathode.
WO 2012001745 and U.S. 2013101907 relate to a metal-air secondary battery with an air cathode made of porous metal. The cathode is described as containing the base material, a carrier, a catalyst and a binder. The catalyst is exemplified as a metal oxide.
U.S. Pat. No. 8,012,633 relates to a secondary metal-air battery with an alkali metal anode and an oxygen (air) cathode. The electrolyte is an aqueous catholyte. The battery contains an ion-selective membrane between the anode and the catholyte (preventing catholyte from entering the anode). Water soluble discharge product is described as stored in the aqueous catholyte. The oxygen cathode is exemplified by reference to U.S. Pat. No. 7,259,126 as having a gas-diffusion layer and a reaction layer. Both the gas-diffusion layer and the reaction layer are described as being made of Teflon and carbon.
U.S. 20130157149 and WO 2011/154869 relate to a rechargeable alkali metal-air battery. The battery contains a molten alkali metal anode, an air cathode and an electrolyte medium between them. An exemplified electrolyte medium is beta-alumina. The air cathode is described as an oxygen (gas) permeable porous substrate having a gas diffusion medium and an oxygen redox catalyst.
U.S. 2011/0195320 relates to an air secondary battery. The battery is report to include a hermetically sealed housing which allows maintenance of a lower than atmospheric pressure of oxygen-containing gas external to the battery cell. This feature is said to facilitate oxygen release from the cell. The air cathode is described as containing a conductive material and may contain a catalyst and is exemplified by a mixture of carbon black, MnO2 and polymer coated on carbon paper.
Certain fuel cells also employ air electrodes. For example, U.S. Pat. No. 6,534,211 relates to a fuel cell having a fuel electrode and an air electrode separated by an electrolyte film. The air electrode is described as having a particular close-packed structure. The air electrode is described as porous and as being made from certain specific mixed metal oxides with certain ranges of particle size which is said to decrease shrinkage on sintering. WO2003/00167 relates to a hybrid fuel cell battery having liquid metal, anodic material, an electrolyte and a cathode. The cathode material is described as including mixed metal, including perovskite-type oxides. In examples the cathode is described as porous. Anodic material is said to include ceramics or ceramics doped with metal and examples of ceramics for anodic material include cerium oxide. In an embodiment the anodic layer is said to have ionic conductivity. The anodic layer in this device is described as between the liquid metal and the electrolyte and is not adjacent the cathode.
The present invention provides improved secondary alkali metal/oxygen batteries which possess high energy density, low cost and both rapid kinetics at the reversible oxygen electrode and, because of the nature of the strategy used for oxygen mediation to and from the cathode, long lifetime.